1. Field of the Invention
The present invention relates to a transmission circuit comprising a multistage amplifier, which transmission circuit is used in communication devices such as mobile phones and wireless LAN devices. The present invention particularly relates to a transmission circuit comprising a multistage amplifier, which transmission circuit is small in size, operates with high efficiency and outputs a transmission signal having high linearity, and also relates to a communication device using the transmission circuit.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to, regardless of a magnitude of an output power thereof, secure a precision of a transmission signal and operate with low power consumption. A transmission circuit used in such communication devices is small in size, operates with high efficiency and outputs a transmission signal having high linearity. Hereinafter, conventional transmission circuits will be described.
One of the conventional transmission circuits is, for example, a transmission circuit which uses a modulation method such as a quadrature modulation method to generate a transmission signal (hereinafter, referred to as a quadrature modulation circuit). Since the quadrature modulation circuit is well known, a description thereof will be omitted. A conventional transmission circuit, which is smaller in size and operates more efficiently than the quadrature modulation circuit, is, e.g., a transmission circuit 500 shown in FIG. 10. FIG. 10 is a block diagram showing a structure of the conventional transmission circuit 500. As shown in FIG. 10, the conventional transmission circuit 500 comprises a signal generation section 501, a phase modulator 502, a regulator 503, a PA 504 and a power supply terminal 505.
In the conventional transmission circuit 500, the signal generation section 501 generates an amplitude signal M and a phase signal P. The amplitude signal M is inputted to the regulator 503. The regulator 503 is supplied with a DC voltage from the power supply terminal 505. The regulator 503 supplies, to the PA 504, a voltage Vc which is controlled in accordance with the inputted amplitude signal M. Typically, the regulator 503 supplies, to the PA 504, the voltage Vc which is proportional to a magnitude of the inputted amplitude signal M.
The phase signal P generated by the signal generation section 501 is inputted to the phase modulator 502. The phase modulator 502 phase-modulates the phase signal P, thereby outputting a phase-modulated signal PM. The PA 504 amplifies the phase-modulated signal PM by using the voltage Vc supplied from the regulator 503. The signal amplified by the PA 504 is outputted from an output terminal as a transmission signal. Here, an output voltage of the PA 504 is referred to as Vout. Note that, the transmission circuit 500 as described above is referred to as a polar modulation circuit.
However, the conventional transmission circuit 500 can not always output a highly precise transmission signal, depending on an offset characteristic of the PA 504. The offset characteristic of the PA 504 will be described with reference to FIG. 11. FIG. 11 shows an example of a variation in the offset characteristic of the PA 504, the variation corresponding to a temperature of the PA 504. This example shows a relationship between the voltage Vc supplied to the PA 504 and the output voltage Vout. It is assumed here that a magnitude of an input voltage (i.e., a magnitude of the phase-modulated signal PM) is fixed. It is known that an offset voltage of the PA 504 varies in accordance with the temperature of the PA 504 as shown in FIG. 11. In other words, the conventional transmission circuit 500 has a problem that since the offset voltage of the PA 504 is not compensated for in accordance with the temperature, a highly precise transmission signal cannot be always outputted.
U.S. Pat. No. 6,998,919 (hereinafter, referred to as Patent Document 1) discloses a transmission circuit 600 which is capable of compensating for the offset voltage of the PA 504 in accordance with the temperature of the PA 504. FIG. 12 is a block diagram showing an exemplary structure of the conventional transmission circuit 600 disclosed in Patent Document 1. As shown in FIG. 12, the conventional transmission circuit 600 comprises the signal generation section 501, the phase modulator 502, the regulator 503, the PA 504, the power supply terminal 505, a temperature measuring section 601 and an offset compensation section 602. The temperature measuring section 601 measures the temperature of the PA 504. The offset compensation section 602 changes the magnitude of the amplitude signal M in accordance with the temperature of the PA 504 measured by the temperature measuring section 601, thereby compensating for the offset voltage of the PA 504.
However, even if the offset voltage of the PA 504 is compensated for in accordance with the temperature of the PA 504, the conventional transmission circuit 600 cannot always output a highly precise transmission signal. The reason for this is that not only the offset voltage of the PA 504 but also a sensitivity characteristic of the PA 504 varies in accordance with the temperature of the PA 504. FIG. 13 shows an example of a variation in the sensitivity characteristic of the PA 504, the variation corresponding to the temperature of the PA 504. FIG. 13 shows a relationship between the voltage Vc supplied to the PA 504 and the output voltage Vout. Here, the magnitude of the input voltage (i.e., the magnitude of the phase-modulated signal PM) is fixed. As shown in FIG. 13, the sensitivity characteristic of the PA 504 varies in accordance with the temperature of the PA 504. Accordingly, there is a problem that even if the offset voltage of the PA 504 is compensated for, the conventional transmission circuit 600 cannot always output a highly precise transmission signal since the sensitivity characteristic of the PA 504 is not compensated for in accordance with the temperature.